Communications from 1978 between the State Department and the US embassy in Wellington show the fishing floats went through radiation testing and were examined by scientists at the then Department of Scientific and Industrial Research (DSIR).

According to a message the embassy sent to the State Department in November 1978, an Ashburton milkman reported an object flying through the sky on October 23.

"John Lovett claims to have been in the paddock on October 22 and is certain the fragment, which was prominently located, was not in the paddock at that time."

A second message from the embassy said it was "suspect" that the ball landed only eight kilometres south of where the 1972 balls landed.

"DSIR's advice ... that the sphere was a silicon-aluminium alloy with a melting point of less than 600 degrees caused doubt that a space object with such a low melting point could survive re-entry ... even if shielded," it said.

"The discovery of a second sphere on November 5, however, seems to increase the likelihood that the objects are indeed space fragments ... "'

Scientists thought "magnetic globules" on the surface of the balls were molten steel. Later that month, they were found to be welded fishing floats.

One of the 1972 "real" space balls is now in the Ashburton Aviation Museum.

Curator Jim Chivers said the US documents were a fascinating insight into the then government's interest.

"The finding of the balls created a great deal of interest from the United States, with much informational data and photographs being requested by their space agency."

He said the level of interest from the US government was never known because all its information was classified "secret and confidential".

"It appears from some of these no longer secret documents that it took quite some work before the hoax balls proved to be just that."

He said the perpetrators would have been pleased with their efforts.

"Had they known that as well as convincing John Lovett that he had found a spaceball, they almost fooled the American space agency as well ... They would have been very pleased," he said.